1. Field of the Invention
The present invention relates to clutch modulating systems, and more particularly to a system for providing modulating control of a power takeoff clutch in an agricultural vehicle.
2. History of the Prior Art
A power takeoff shaft has been a conventional item on certain agricultural vehicles such as tractors for many years. The power takeoff shaft provides a means for coupling the engine of the tractor to drive agricultural implements being towed by or otherwise coupled to the tractor. The power takeoff shaft is typically coupled to the tractor engine through a clutch which controls the application of engine power to the shaft and thereby to the agricultural implements. On some tractors a separate brake may be provided for locking out rotation of the power takeoff shaft simultaneously with disengagement of the clutch.
An example of a tractor transmission system which includes a hydraulically operated power takeoff clutch and a hydraulically operated power takeoff brake is provided by U.S. Pat. No. 4,318,305 of Peter D. Wetrich et al, issued Mar. 9, 1982 and commonly assigned with the present application. The system described in the Wetrich et al patent requires the presence of pressurized hydraulic fluid to maintain engagement of the power takeoff clutch. Upon loss of hydraulic fluid pressure, either intentionally through manipulation of the power takeoff control lever or by accident such as in the case of hydraulic leak or failure, the power takeoff clutch becomes disengaged and a power takeoff brake simultaneously becomes actuated so as to lock the power takeoff shaft to the transmission housing to prevent rotation of the shaft. The automatic disengagement of the power takeoff clutch upon loss of hydraulic fluid pressure is a desirable safety feature. The resulting uncoupling of the agricultural implements from the tractor engine in the event the tractor stalls or the engine is intentionally turned off insures that upon restarting an attached implement does not immediately begin operating so as to injure the operator or another who may be close to or working on the implement. The operator is required to move the power takeoff control lever into the engaged position each time the tractor is started before the implements attached to the power takeoff shaft can be operated.
When operating tractors of various types including those having a transmission system like that shown in the Wetrich et al patent, it is desirable to be able to move the power takeoff control lever quickly into the engage position. This is particularly true during operations where the operator is busy with other controls. At the same time rapid movement of the power takeoff control lever into the engage position may result in excessively rapid engagement of the power takeoff clutch, resulting in damage to the clutch or to the implements coupled to the power takeoff shaft. Accordingly, it would be desirable to be able to move the power takeoff control lever into the engage position very quickly without danger of damage to either the tractor or the implements being driven by the power takeoff shaft.
At the same time it is desirable to be able to manually modulate the power takeoff control lever so as to vary the extent of engagement of the power takeoff clutch. Many types of agricultural operations require some coupling of the power takeoff shaft to the tractor engine which is less than total engagement. Nevertheless, many tractors feature only an on-off type of control in which the power takeoff shaft is either positively coupled to or is uncoupled from the tractor engine.
Another feature which is desirable with the power takeoff portion of a tractor is the ability to deliver shocks to the power takeoff drive line when necessary. Occasionally the implements being driven by the power takeoff shaft may become clogged or stalled or otherwise temporarily immobilized. In such situations the delivery of shocks to the power takeoff drive line will frequently aid in untangling such implements. However, such feature is in contradiction to the more prevalent desire to provide for gradual engagement of the power takeoff clutch so as to prevent excessively rapid engagement of such clutch during most operating conditions.
The prior art includes various examples of systems which vary the application of pressurized fluid to a clutch, transmission or similar device. Such examples include those provided by U.S. Pat. Nos. 3,722,646, 3,468,194, 4,093,051, 3,603,344, 3,348,644, 3,438,469 and 3,352,392. Still other examples utilizing valves and other devices to provide a desired delaying action in the operation of clutches, transmissions and similar devices include U.S. Pat. Nos. 2,807,968, 2,939,557, 3,656,600, 3,670,599, 3,674,121, 3,707,891, 3,709,065, 3,882,980, 3,972,401, 3,990,553, 4,000,795, 3,085,834, 4,096,882, 4,150,737, 3,621,955, 3,850,273, 3,863,523, 3,998,111 and 4,111,071.
None of the patents noted above provide a system comprising the various desirable features previously noted. Thus, such systems do not provide a modulating system well suited for controlling a power takeoff clutch or similar arrangement in which the degree of clutch engagement can be effectively modulated while at the same time enabling the control lever to be quickly moved into the engage position without danger of damage to the system. Additional desirable features in combination therewith which are not shown or suggested by the art noted above include the automatic disengagement of the clutch upon loss of hydraulic fluid pressure and the ability to apply controlled shock loads to the drive line in combination with the other features.